This invention relates to an admixture for incorporation into hydraulic cement, mixtures that optionally include SCM""s, such as concrete, grouts, mortars, patching compounds and dry mixes for making the same, to produce cementitious compositions having high fluidity and high early compressive strength.
In many applications, such as pre-cast concrete, it is desirable that the cementitious composition used has sufficient fluidity so as to flow freely through and around the reinforcement structure to fill out the mold and level off at the top of the mold. Typically, the mold needs to be vibrated to facilitate the leveling of the concrete, which uses energy and time. Equally important as this need for high flowability, however, is the need for high early strength. The term xe2x80x9cearly strengthxe2x80x9d refers to the compressive strength of the cementitious mass within the first twenty-four hours after mixing the cement mixture with water. Since the goal in this industry has been to reduce the time for demolding as much as possible, it has been particularly desirable to provide cementitious compositions that have a compressive strength within 13 and 17 hours that allows for demolding.
Certain chemical additives have been used in the art to improve the flowability and/or early strength of cementitious compositions. Calcium chloride, for instance, has been used as an effective and economic accelerator especially at low temperatures. Nevertheless, the conventional use of calcium chloride has been restricted because it promotes the corrosion of metal reinforcement at the levels at which it is included in the cementitious composition. Furthermore, at the lower application levels at which the calcium chloride is not corrosive, it is ineffective. Other inorganic salts as well as amines have also been used as accelerators, however they all provide little, if any, benefit on the fluidity of the cementitious composition. On the other hand, increased fluidity can be achieved by using large amounts of water in the concrete but the resulting cementitious structure exhibits insufficient compactness, is prone to cracking and has poor compressive strength.
Another method for increasing the fluidity of the cementitious composition is to add superplasticizers or high range water-reducers (HRWR""s) like sulphonated melamine- or naphthalene-formaldehyde polycondensates or ligninsulphonate-based admixtures. More recently, HRWR""s based on water-soluble polycarboxylic acid salts, e.g., alkoxylated copolymers of acrylic acid and acrylic esters have been proposed for imparting high water reduction at lower dosages. These copolymers typically have a poly(oxyethylene) chain connected to the acrylic backbone of the copolymer via ester bonds and have conventionally been used as dispersing agents. The strong water reducing capacity of both the conventional naphthalene/melamine superplasticizers and the new generation superplasticizers can be used to effectively lower the ratio of the water to the cementitious material in a concrete mixture and thus provide increased compressive strength at both early and later ages. However, although these superplasticizers can be used to increase the early compressive strength of the cementitious composition, there is a need in the art to further increase the early compressive strength of the cementitious composition while still providing good flowability.
The present invention is an admixture for cementitious compositions that provides both good flowability and high early strength as is desired in the art. In addition, the admixture of the invention does not possess the drawbacks of conventional additives used in cementitious compositions. For example, the admixture is chloride free and thus does not have the tendency to corrode metal equipment. The admixture used in the invention also lowers the water-to-cementitious (W/CM) ratio of the cementitious composition. Furthermore, the admixture uses a low dose of organic accelerants, especially compared to the amount of inorganic accelerants conventionally used in the art.
The admixture of the invention includes a superplasticizer comprising a water-soluble modified acrylic copolymer and at least one accelerant selected from the group consisting of salts of thiocyanic acid, water-soluble alkanolamines, ethylene oxide adducts of ethylenediamine, and morpholine derivatives. Preferably, the accelerant includes a salt of thiocyanic acid and water-soluble alkanolamine. More preferably, the accelerant includes each of a salt of thiocyanic acid, a water-soluble alkanolamine, ethylene oxide adducts of ethylenediamine, and morpholine derivatives. The combination of the superplasticizer and the accelerant used in the invention surprisingly has a synergistic effect on the early compressive strength of the cementitious composition and also enhances the fluidity of the cementitious composition allowing for increased reduction in water demand and the use of a lower W/CM ratio in practical applications.
The superplasticizer and accelerant are included in a cementitious mixture comprising a hydraulic cement and optionally a supplementary cementitious material (SCM) and is combined with aggregate and water to produce the cementitious composition. The water is included in an amount sufficient to effect hydraulic setting of the cementitious composition. The hydraulic cement is preferably Portland cement. The SCM is preferably fly ash, ground granulated blast furnace (i.e. slag), and/or silica fume and is more preferably fly ash. The accelerant preferably includes an alkali metal, ammonium or alkaline earth metal salt of thiocyanic acid; a water-soluble poly(hydroxyalkyl)polyethyleneamine; ethylene oxide adducts of ethylenediamine; and a composition of morpholine derivatives. More preferably, the salt of thiocyanic acid is selected from the group consisting of sodium, potassium, ammonium, calcium and magnesium thiocyanates (e.g. sodium thiocyanate); the water-soluble poly(hydroxyalkyl)polyethyleneamine is tetra(hydroxyethyl)ethylenediamine; the ethylene oxide adducts are selected from the group consisting of sym-dihydroxyethylethylenediamine, unsym-dihydroxyethylethylenediamine, aminoethylethanolamine, trihydroxyethylethylenediamine and tetrahydroxyethylethylenediamine (e.g. tetra(hydroxyethyl)ethylenediamine); and the morpholine derivatives are selected from the group consisting of morpholineaminodiglycol, morpholinyldiglycol, dimorpholinoethane, dimorpholinediglycol, methoxymethylmorpholine, morpholine ethaneamine and morpholine ethanol. The superplasticizer and the at least one accelerant are typically present in amounts sufficient to provide an early compressive strength at 13 hours after application that is greater than or equal to 200% percent of the early compressive strength of a comparable composition that does not include the superplasticizer or accelerant. Moreover, the at least one accelerant is typically present in an amount sufficient to generally provide an early compressive strength at 13 hours after application that is greater than or equal to 30% percent, more preferably greater than or equal to 50%, of the early compressive strength of a comparable composition that does not include the accelerant. The accelerant typically includes the salt of thiocyanic acid in an amount from about 0.005% to about 1% by weight; the water-soluble poly(hydroxyalkyl)polyethyleneamine in an amount from about 0.004% to about 0.5% by weight; the ethylene oxide adducts of ethylenediamine in an amount from about 0.0001% to about 0.08% by weight; and the morpholine derivatives in an amount from about 0.0004% to about 0.35% by weight, based on the total cementitious content (i.e. cement and SCM). The water-soluble modified acrylic copolymer is present in an amount from 0.03% to about 0.4% by weight based on the total cementitious content. In a preferred embodiment, the cement mixture preferably includes Portland cement in an amount from about 50% to about 100% by weight and the SCM in an amount from greater than 0% to about 50% by weight.
In a preferred embodiment, the present invention includes a cementitious composition comprising (1) a cementitious mixture comprising Portland cement in an amount from about 60% to about 100% by weight based on the total cementitious content; fly ash in an amount from greater than 0% to about 40% by weight based on the total cementitious content; a water-soluble modified acrylic copolymer superplasticizer in an amount from 0.03% to about 0.4% by weight based on the total cementitious content; an accelerating composition comprising a salt of a thiocyanic acid in an amount from about 0.005% to about 1% by weight, a water-soluble poly(hydroxyalkyl)polyethyleneamine in an amount from about 0.004% to about 0.5% by weight, ethylene oxide adducts of ethylenediamine in an amount from about 0.0001% to about 0.08% by weight; and morpholine derivatives in an amount from about 0.0004% to about 0.35% by weight, based on the total cementitious content; (2) aggregate in an amount from greater than 0% to about 80% by weight based on the total cementitious content; and (3) a sufficient amount of water to effect hydraulic setting of the cementitious composition.
The present invention also includes a method for obtaining high early strength and increasing the flow of a cementitious composition, comprising the steps of combining a hydraulic cement; an optional SCM; a superplasticizer comprising a water-soluble modified acrylic copolymer; at least one accelerant selected from the group consisting of salts of thiocyanic acid, water-soluble alkanolamines, ethylene oxide adducts of ethylenediamine, and morpholine derivatives, aggregate and a sufficient amount of water to effect hydraulic setting of the cementitious composition. Preferably, the cementitious composition is formed by sequentially adding the aggregate, 50-80% of the water, the cementitious material (cement and SCM), the remaining water and the admixture.
These and other features and advantages of the present invention will become more readily apparent to those skilled in the art upon consideration of the following detailed description, which describe both the preferred and alternative embodiments of the present invention.